1. Field of the Invention
The present invention relates to a water base ink set for ink-jet recording in which at least a black ink and a color ink are used in combination, and a method for producing the water base ink set.
2. Description of the Related Art
Dye water base inks, containing a water-soluble dye as the coloring agent which is excellent in handling performance, and which hardly deposit precipitates and exhibit satisfactory color vividness or brightness, have been hitherto widely used for the ink-jet recording. At the same time, it is more demanded that the ink-jet recording is performed on a regular paper rather than an exclusive ink-jet paper, thereby decreasing the running cost for printing operation. In general, the water base ink is easily absorbed in the regular paper and thus likely to blur on the regular paper. Accordingly, when an ink-jet recording is performed on the regular paper with such a dye water base ink, the problem of the bleeding is likely to occur. The bleeding refers to such a phenomenon that the printing quality deteriorates at a portion in which different colors are adjacent to each other because the inks having different colors are blurred and mixed with each other at the portion. In particular, the bleeding between black and color inks is a major cause of the deterioration of printing quality.
In order to prevent the bleeding and thus improve the printing quality, it has been suggested, for example, to use a water-immiscible black pigment as the coloring agent so as to control the movement of color material of the black ink on the paper surface (the specification of U.S. Pat. No. 5,745,140). On the other hand, for the purpose of improving the permeability of the dye water base color ink to the inside of the paper, it has been suggested to blend a permeating agent such as a water-soluble polyvalent alcohol alkyl ether and a surfactant into the ink in order to decrease the surface tension of the ink (Japanese Patent Application Laid-open No. 8-283631).
The black pigments used in the black ink for ink-jet recording are generally classified into the following two types by the way the black pigments are dispersed in the water. One type is a dispersing agent-aided dispersion pigment with which a dispersant agent such as a surfactant and a water-soluble high molecular compound are used. The other type is a self-dispersible pigment in which a surface oxidation treatment or the like is applied to the pigment particles such as carbon black so that the surfaces of the pigment particles are charged to have a negative electric charge, and thus the pigment particles can be dispersed in the water by themselves.
There is, however, a following problem associated with the dispersing agent-aided dispersion pigment. The dispersing agent used for preparing a black ink with the dispersing agent-aided dispersion pigment facilitates the action of the black ink to permeate into the paper, which in turn causes any non-uniform blurring. In addition, when the water content in the ink is decreased after the evaporation of water, the dispersing agent causes the ink to solidify. On the other hand, the self-dispersible pigment, in which the dispersion stability relies only on the electrostatic repulsive force, has a following problem. The dispersion of the self-dispersible pigment is likely to become unstable when the ink co-exists with salts, and when the self-dispersible pigment makes contact with a dye color ink having a structure of salt, the aggregation of pigment occurs, which in turn causes a clog-up of the nozzle and consequently a discharge failure.
In addition, when a permeating agent such as water-soluble polyvalent alcohol alkyl ether and a surfactant are used to adjust the permeability of the ink for ink-jet recording into the paper, the following problem may occur although the cause therefor is unknown. Namely, the aggregation is likely to occur when a black ink and a color ink make contact with each other. Therefore, there is a possibility in some cases that the aggregation of pigment occurs to cause a clog-up of the nozzle and consequently the discharge failure.
As explained above, when the ink-jet recording is performed on a regular paper as the recording paper, it has been quite difficult to realize both of the suppression of bleeding and the stable discharge.